1. Field of the Invention
This invention relates to a method for forging IN 706 components.
2. Description of the Related Art
Inconel Alloy 706 is a nickel based super alloy having high strength up to about 1200.degree. F. and resistance to embrittlement. As a result, it has found use in a number of gas turbine components. The IN 706 alloy has the composition shown below, from the AMS specification 5703B.
______________________________________ Element Weight Percent ______________________________________ carbon 0.06 max. manganese 0.35 max. phosphorus 0.35 max. sulfur 0.015 max. chromium 14.5 to 17.5 nickel 39 to 44 niobium 2.5 to 3.3 titanium 1.5 to 2 aluminum 0.4 max. boron 0.006 max. copper 0.3 max. iron balance ______________________________________
In "Properties of a Fabricable, High Strength Alloy," H. L. Eiselstein, Metals Engineering Quarterly, November 1971, pages 20-25 two heat treatments are disclosed for the IN706 alloy. A first heat treatment has three steps, annealing at 1800.degree. F. for 30 minutes, air cooling to 1550.degree. F., holding for 3 hours, air cooling to 1325.degree. F., holding for 8 hours, furnace cooling to 1150.degree. F., holding for 8 hours, and air cooling. An alternative two step heat treatment eliminates the 1550.degree. F. step from the three step treatment. Eiselstein discloses higher tensile strength, elongation, and toughness for the two step treatment.
A conventional thermomechanical treatment for nickel base superalloys provides for air cooling to room temperature after the forging process. The forging is machined to remove surface irregularities introduced during the forging process, and solutioned by heating to a temperature where precipitate phases are dissolved and placed into solid solution. Next the forging is water or oil quenched, and aged by heating the billet at specific temperatures to precipitate phases, such as gamma prime, which develop the desired mechanical properties in the alloy.
The forged surface has a number of surface irregularities that can act as stress raisers that initiate cracking during the quenching step. As the forging size increases, the temperature difference between the surface and center of the forging increases, and the surface stresses caused by the quenching step increase. As a result, the forged surface is machined smooth prior to the solutioning and quenching steps to prevent cracks from forming in the surface of the forging during the quench.
I have discovered the conventional forging process has several disadvantages when performed on IN 706 large forgings, such as rotor wheels in gas turbines. Air cooling after forging permits the grains in the interior of the large forging to recrystallize and grow to large sizes. In addition, reheating to solution treat the large forging after it has been cooled to room temperature requires several hours at about 1800.degree. F. causing further increase in grain size.
It is well known that grain growth reduces cyclic fatigue life, and therefore a fine grain size is desirable to provide improved low cycle fatigue life. A fine grain size also permits improved ultrasonic inspection of the alloy members, so that smaller defects can be detected during inspection. Therefore, a fine grain size is desired to improve service life of IN 706 alloy components.
It is an object of this invention to provide a forging process for IN 706 that minimizes the time at solution and forging temperatures to minimize grain growth.
Another object of this invention is to provide large IN 706 forgings having a finer grain size.
Another object of this invention is to provide a simplified forging process for IN 706 with a reduced number of steps.
Another object of this invention to provide a forging process for IN 706 that minimizes the formation of embrittling grain boundary phases.